1. Field of the Invention
The present invention relates to an in-cell touch panel type liquid crystal display, and more specifically to a liquid crystal display and a method of driving the same capable of accurately sensing whether or not a liquid crystal display panel is touch and improving the reliability of a touch sensor circuit inside the liquid crystal display panel.
2. Discussion of the Related Art
A liquid crystal display (LCD) displays an image by controlling light transmitted through a liquid crystal layer using an electric field generated in response to a video signal and applied to the liquid crystal layer. Because the liquid crystal display is a thin, small-sized flat panel display device with low power consumption, the liquid crystal display has been used in personal computers such as notebook PCs, office automation equipment, audio/video equipment, and the like. In particular, because an active matrix type liquid crystal display includes a switching element in each liquid crystal cell that is actively controlled, the active matrix type liquid crystal display is advantageous in displaying a moving picture. A thin film transistor (TFT) has been mainly used as the switching element of the active matrix type liquid crystal display.
In general, an active matrix type liquid crystal display converts digital video data into an analog data voltage based on a gamma reference voltage. As shown in FIG. 1, the analog data voltage is then applied to a data line DL while a scan pulse is applied to a gate line GL. The scan pulse activates a thin film transistor TFT connected thereto, thereby charging a liquid crystal cell Clc with the data voltage on the data line DL. The thin film transistor TFT includes a gate electrode connected to the gate line GL, a source connected to the data line DL, and a drain electrode commonly connected to a pixel electrode of the liquid crystal cell Clc and one electrode of a storage capacitor Cst. A common voltage Vcom is applied to a common electrode of the liquid crystal cell Clc. When the scan pulse is applied to the gate line GL, the thin film transistor TFT is turned on, and a channel is formed between the source and drain electrodes of the thin film transistor TFT. Hence, a voltage on the data line DL is supplied to the pixel electrode of the liquid crystal cell Clc. Additionally, when the thin film transistor TFT is turned on, the storage capacitor Cst is charged with the data voltage applied through the data line DL, thereby keeping the voltage level of the liquid crystal cell Clc constant. The orientation of liquid crystal molecules in the liquid crystal cell Clc changes due to an electric field generated between the pixel electrode and the common electrode, thereby changing the incident light. Because the liquid crystal display is generally a light receiving element (i.e., liquid crystals do not generate light), the liquid crystal display adjusts luminance of the screen using light produced by a backlight unit formed in the rear of a liquid crystal display panel.
Recently, a touch screen panel attached to the liquid crystal display has been proposed. The touch screen panel generally attached on the liquid crystal display is a user interface that detects changes in electrical characteristics at a touch position where an opaque object, such as a finger or a pen, contacts the touch screen panel. When a user's finger or a touch pen contacts the screen of the touch screen panel of the liquid crystal display, a display device detects the information of the touch position and uses the information for various applications.
However, the touch screen panel of the liquid crystal display generally increases the manufacturing cost of the liquid crystal display, may reduce the manufacturing yield due to the added process of attaching the touch screen panel to the liquid crystal display, may reduce brightness of the liquid crystal display, increases the thickness of the liquid crystal display, and the like.
To solve the above-described problems, rather than attaching a touch screen panel, an in-cell touch panel type liquid crystal display has been proposed. An in-cell touch panel type liquid crystal display includes a touch sensor circuit including a sensor thin film transistor (TFT) formed inside the liquid crystal cell Clc of the liquid crystal display. As shown in FIG. 2, the touch sensor circuit includes a sensor TFT that changes a light current “i” depending on changes in the amount of external light coming into the panel, a sensor capacitor Cst2 that stores a charge generated by the light current “i,” and a switching TFT that switches on and off an output of the charge stored in the sensor capacitor Cst2. A bias voltage Vbias is supplied to a gate electrode of the sensor TFT and is set at a voltage level equal to or smaller than a threshold voltage of the gate electrode of the sensor TFT.
In the touch sensor circuit shown in FIG. 2, the light current (i) of a sensor TFT in a touch area is larger than the light current of a sensor TFT in a non-touch area when the outside environment is darker than the backlight (e.g., indoors). On the other hand, the light current (i) of a sensor TFT in a touch area is smaller than the light current of a sensor TFT in a non-touch area when the outside environment is brighter than the backlight (e.g., outdoors). In either circumstance, a light sensing signal produced in the touch area is different than a light sensing signal in the non-touch area. Accordingly, the liquid crystal display detects the touch position information based on the light sensing signals of the touch sensor circuit. However, the in-cell touch panel type liquid crystal display has the following drawbacks.
Because the related art in-cell touch panel type liquid crystal display detects the touch position based on only a relative difference between the light currents flowing in the sensor TFTs, detection of whether or not the liquid crystal display panel is actually touched cannot be determined accurately. For example, FIG. 3A shows that if external light produces a shadow from the user's fingers in a strong illuminance environment (e.g., outdoor environment) without touching the liquid crystal display, the related art in-cell touch panel type liquid crystal display may not be able distinguish from an actual touch condition shown in FIG. 3B.
Further, because the sensor TFT needs to continuously receive a DC driving voltage Vdrv with a high potential through the gate electrode of the sensor TFT for the above-described light sensing operation, the sensor TFT may be degraded when the sensor TFT is driven for a long period of time. Because the degradation of the sensor TFT reduces the output characteristics of the touch sensor circuit to cause an error of the output light sensing signal, the reliability of the touch sensor circuit is reduced. In other words, if the sensor TFT is degraded, the touch sensor circuit may fail to output a sensing signal indicating that the liquid crystal display panel has been touched even if the user touches the liquid crystal display panel with his finger. Conversely, the touch sensor circuit may output a sensing signal indicating the liquid crystal display panel has been touched even if the user does not touch the liquid crystal display panel with his finger.